1. Field of the Invention
This invention relates to a magnetic recording medium, more particularly, to a magnetic disc which includes a substrate having coated on at least one surface thereof, in sequence, a magnetic recording layer, a protective coating, and a lubricant coating. The magnetic recording medium of this invention is particularly useful in magnetic recording devices such as a magnetic disc device based on a contact-start-stop (CSS) system. This invention also relates to a process for the production of the magnetic recording medium.
2. Description of the Prior Art
Heretofore, various types of the magnetic recording media have been developed for use in magnetic disc devices for data handling and other applications. They generally include a disc-like substrate of aluminum, similar light metals, or their alloys at least one surface of the substrate having a magnetic recording layer.
A typical example of the prior art magnetic recording media can be found in FIG. 1, in which reference number 1 is an aluminum substrate. In the illustrated recording medium, the substrate 1 has an alumite or anodized aluminum coating 2 effective for enhancing the physical and chemical properties of the underlying aluminum substrate and adhesion of a magnetic recording layer 3 to the substrate. There are two types of magnetic recording layers: One is a discontinuous magnetic layer, having dispersed therein binding agents or binders such as synthetic resins, which can be produced by coating a mixture of magnetic powders and binders on the substrate by means of a spin coater. The other is a continuous magnetic layer containing no binder. The continuous magnetic layer can be produced, for example, by depositing magnetic materials such as iron and iron-cobalt alloys onto the substrate by well-known techniques such as sputtering, vacuum evaporation, plating, and electrodeposition.
Binder-free continuous magnetic recording layers are widely used in the art, since they can be simply produced and have excellent magnetic characteristics and high recording density. Of the various magnetic materials, ferrite oxide is particularly useful for the magnetic recording layer because of its hardness and resistance to corrosion.
Magnetic recording media with the above-described continuous magnetic layer, however, are disadvantageous when used in CSS system-based magnetic disc devices. In these magnetic disc devices, the magnetic head is generally picked up from and maintained over the surface of the magnetic layer during writing or recording and reading of information. At the start and stop of the rotation of the recording media, however, a slider of the magnetic head contacts and slips on the surface of the magnetic layer. Repeated contact and slippage of the slider with the magnetic layer results in wear on the surface in a head rounding zone of the magnetic recording media, spread of fine powders of the worn magnetic layer over the surface of the recording media, and consequently crushing of the magnetic head or briefly "head crush" and destruction of the stored information. It is therefore desirable to increase the resistance to wear of the magnetic recording media.
Many useful means and methods have been already proposed to prevent or decrease wear of the magnetic recording layer during operation of CSS system-based magnetic disc devices. One of them is the formation of a protective coating. For example, Japanese Unexamined Patent Publication (Kokai) No. 52-20804 discloses a magnetic recording medium having a protective coating of selected polysilicate. Other suitable protective coatings are those of synthetic resins or silicon dioxide. The formation of the protective coating is illustrated in FIG. 2. In FIG. 2, the layer constitution is identical with that of FIG. 1 except that a protective coating 4 is further coated as a top layer. This type of protective coating, however, cannot completely prevent wear of the underlying magnetic layer. The protective coating has a smooth surface due to the mirror surface of the alumite coating on the aluminum substrate. The smooth surface of the protective coating causes slippage of the magnetic head. This results wears down the coating and again which induces head crush.
As another means to decrease the wear of the magnetic recording layer, Japanese Unexamined Patent Publication (Kokai) No. 54-161909 suggests the use of a combination of a liquid lubricant such as perfluoroalkylpolyether and a solid lubricant such as telomer of tetrafluoroethylene on the magnetic recording layer. These lubricants may also be used on the protective coating, if desired. The lubricant coating formed on the magnetic layer or on the protective coating can smooth the slippage of the magnetic head for a short time, however, with time, the lubricant coating becomes thinner and decreases in lubricant action. As a result, the problems encountered in the previously described cases also occur. Further, an increase in the amount of the lubricant used would cause undesirable adhesion of the magnetic head to the surface of the magnetic recording layer. This adhesion of the head must be avoided, since it results in head crush or destruction of the supporting means for the magnetic head.
Japanese Unexamined Patent Publication (Kokai) No. 57-20925 teaches the formation of small cylindrical projections on the magnetic layer or the overlying protective coating. An example of a protective coating having small cylindrical projections can be found in FIG. 3A (cross-sectional view) and FIG. 3B (perspective view). From these figures, it will be understood that a plurality of cylindrical projections 5 having the same height are disposed on the protective coating 4. In FIG. 3A, 1 is an aluminum substrate, 2 is an alumite coating, and 3 is a magnetic recording layer. In this case, the lubricant is impregnated into the voids or gaps formed between the adjacent projections. This effectively eliminates the problems of adhesion of the magnetic head. However, with time, the centrifugal force during the rotation of the recording medium destroys the uniform distribution of the lubricant. Consequently, the lubricant coating at the outer portion of the recording medium becomes thicker. The lubricant frequently falls from the surface of the protective coating. It is, therefore, difficult to use the recording medium for a long period without the defects described above.
On the other hand, Japanese Unexamined Patent Publication (Kokai) No. 56-22221 teaches the formation of small cylindrical recesses on the protective coating of the magnetic recording medium. This is illustrated in FIGS. 4A and 4B, from which it is apparent that a plurality of recesses 6 having the same depth are randomly formed on the protective coating 4. The recesses 6 can effectively retain the lubricant, but they cannot prevent the decrease in the resistance to wear of the protective coating due to its smooth surface. In addition, the size of the recesses is on the order of 10 .mu.m, since the recesses are produced through a masking process. In practice, recesses having a size on the order of 1 .mu.m cannot be produced.
Recently, S. Otaki, one of the inventors of the present case, and four others achieved impregnation of a lubricant into a porous protective coating formed from thermoplastic resins (Cf. Japanese Unexamined Patent Publication (Kokai) No. 58-200431). The impregnated protective coating has excellent resistance to wear, but due to use of thermoplastic resins such as phenol, melamine, and epoxy in the formation of the porous protective coating, shows less CSS strength or mechanical strength.